Document WO 97/27102 describes a method and apparatus for removing a propeller assembly from an opening of a floating vessel, wherein the propeller assembly is designed to close an opening in the vessel hull when the propeller assembly is in its mounted position. According to this prior art, a watertight hoisting chamber is provided around the propeller assembly and inside that hoisting chamber a drive shaft is provided, which leads to a drive motor positioned in the vessel and outside of the hoisting chamber. When the known propeller assembly is to be removed, the drive shaft is removed first. Then its passage through the hoisting chamber wall is closed in a watertight manner. After that the propeller assembly is connected to hoisting means and then a flange of the propeller assembly is loosened. Then, the propeller assembly can be lifted from the hoisting chamber. The propeller assembly is also known as thruster. The method according to the prior art requires assembly/disassembly work which has to be carried out under water.
Further, large vessels can have a draught which is up to 20 meters under water surface level. In this case, considerable forces act to push the thruster upwards and inside the vessel due to the difference in pressures between the water pressure outside and the air pressure inside the vessel. With large thrusters, the forces pushing up the thrusters which are not compensated by thruster weight may reach up to 2000 kN. When the fixing screws of the flange are removed, the propeller assembly or thruster is lifted by these forces in uncontrolled manner. Having such a heavy mass which moves in uncontrolled manner is dangerous. Further, loosening of the fixing of the flange while under these forces is difficult.
Document WO 20111279878 A1 describes a method of maintenance of a unit arranged in the watertight hoisting chamber and closing an opening in a vessel hull, wherein the unit is adapted to extend into the water below the floating vessel. The method comprises the steps of clamping the unit in its mounted position by clamping means, releasing fixing means which fix the unit into its mounted position while holding the unit clamped in its position and at least partly flooding the hoisting chamber, then releasing the clamping of the unit and hoisting the unit away from its mounted position. A clamping means is used to clamp the unit in its mounted position before the fixing means, which are normally used for mounting the unit into its position, are removed. In this way, the opening can be held closed and the unit is held in position, so that the work for releasing the fixing means can be carried out while it is still dry in the hoisting chamber. After the fixings are removed, the hoisting chamber is at least partly flooded either by provision of an extra valve for flooding the chamber or by a controlled release of the clamping of the unit. When the clamping is released, the water in the at least partly flooded hoisting chamber puts some pressure on the unit from the vessel inside, so that the pressure differences at the unit between inside and outside the vessel are reduced. Therefore, the forces pushing up the unit can be reduced. Even if the method is beneficial as such there has emerge some need to further develop the method.
In view of the above prior art, it is the object of the invention to provide an improved method and apparatus for maintenance of a unit which passes through and closes an opening in the vessel hull while the vessel is floating.